1. Field of the Invention
The present invention relates to materials for magnetoresistive sensors used for a magnetic head, particularly a reproducing head, a position sensor, and an angle sensor.
2. Description of the Related Art
Although a Ni--Fe alloy thin film (permalloy thin film) is conventionally used as a magnetoresistive (MR) material, the permalloy film has a rate of change in resistance of 2 to 3%. In the future, in order to comply with the requirements to narrow the track of a magnetic head and increase the resolution of a magnetic sensor, a magnetoresistive material having a higher rate of change in resistance (MR ratio) is demanded.
The phenomenon referred to as "giant magnetoresistive (GMR) effect" has recently been found in Fe/Cr or Co/Cu multilayer thin films (refer to M. N. Baibich et al., Physical Review Letters, Vol. 61 (1988), p2472, D. H. Mosca et al., Journal of Magnetism and Magnetic Materials, Vol. 94 (1991), p L1). It is considered that, in such thin films, spin-dependent scattering caused by conduction electrons located in the interface between Fe and Cr or Co and Cu contributes to the giant magnetoresistive effect. These films basically differ from conventional Ni--Fe thin films in the generation mechanism of the magnetoresistive effect. Although a MR ratio of 10% or more is obtained from these thin films, the need for the films to have a multilayer structure complicates the production process therefor.
Thereafter, the same giant magnetoresistive effect was observed in a single-layer thin film in which ultrafine Co, Fe or Ni grains (grain size: a few nm) are precipitated in a Cu or Ag matrix, as shown in FIG. 6 (for prior art using Co, refer to A. E. Berkowitz et al., Physical Review Letters, Vol. 68 (1992), p3745, J. Q. Xiao et al., Physical Review Letters, Vol. 68 (1992), p3749).
In these materials, the interfaces of Co grains and the Cu(Ag) matrix contribute to the giant magnetoresistive effect. Therefore, if the number of the Co grains precipitated can be increased while maintaining the size of ultrafine grains, i.e., if the Co content can be increased, the MR ratio is increased due to an increase in the total area of the interfaces. FIG. 7 shows changes in the rate of change in resistance (MR ratio) with the volume fraction of ferromagnetic grains. The changes can be theoretically predicted with a constant grain size (refer to S. Zhang et al., Applied Physics Letters, Vol. 61 (1992), p1855).
A conventional Cu--Co alloy has the tendency that the grain size increases as the Co content increases.
There is also the tendency that the rate of change in resistance decreases as the grain size of ferromagnetic Co grains increases, as shown in a theoretical curve of FIG. 8 (refer to S. Zhang et al., Applied Physics Letters, Vol. 61 (1992), p1855).
Because an increase in the Co (ferromagnetic material) content results in increased grain size, and increased grain size results in decreased resistance, it is therefore impossible to improve the giant magnetoresistive effect by only increasing the amount of Co (ferromagnetic material).
FIG. 9 shows the Co-content dependence of the rate of change in resistance (MR ratio) of a Cu--Co alloy sputtered thin film. FIG. 9 indicates that the rate of change in resistance has a peak at the Co content of about 20 at %, and decreases as the Co content increases from about 20 at %.
There is also a problem in that if the grain size of ferromagnetic grains increases, the magnetization process is governed by the crystal magnetic anisotropy of the ferromagnetic grains, thereby increasing hysteresis.
Such a granular GMR material also has another problem with an alloy having a high Cu or Ag content (a low content of ferromagnetic metal) in that, since the respective ferromagnetic grains are spaced and thus magnetically separated, the alloy exhibits a superparamagnetic behavior and a gradual change in magnetization with an external magnetic field, and magnetization is thus slowly saturated. Namely, such an alloy requires a very high magnetic field (saturation magnetic field Hs) for saturation of the resistance change, which is defined as shown in FIG. 10.
Since electric resistance depends upon the relative angle between adjacent ferromagnetic grains with a nonmagnetic matrix therebetween, an overall change in magnetization with an external magnetic field corresponds to a change in electric resistance with the external magnetic field.
The conventional material thus has a problem in that the sensitivity of resistance changes with an external magnetic field is significantly low.